1. Field of the Invention
This invention relates to an intraocular length measuring instrument, in which a beam of light is split into two beams of light, one beam of light is guided to an optical path length variable member disposed at a reference optical path as a reference beam of light and adapted to vary an optical path length of the reference optical path, the other beam of light is guided to an intraocular object to be measured via a measuring optical path as a measuring beam of light, and the reference reflection light from the optical path length variable member and a measuring reflection light from the intraocular object to be measured are caused to be interfered with each other to obtain an interference signal, thereby obtaining a position of the intraocular object. As a result, obtaining the intraocular length.
2. Brief Description of the Prior Art
As a typical example of an intraocular length measuring instrument, there is heretofore known one disclosed in U.S. Pat. No. 5,141,302. This instrument includes a measuring optical path formed via an eye to be tested, and a reference optical path formed within the instrument, light reflected by an intraocular object to be measured after being passed through the measuring optical path and light passed through the reference optical path being caused to be interfered with each other, a reference mirror disposed at the reference optical path being moved to vary the optical path length of the reference optical path, interference fringes being generated by the above interference. Then, a position of the intraocular object to be measured is found with reference to the optical path length of the reference optical path where the interference fringes are generated. A position of a cornea is found by other optical means.
However, in this instrument, since a movement of the reference mirror is a reciprocal linear motion, it is difficult to reciprocate the reference mirror at a high speed repeatedly. Furthermore, the speed of its movement in a whole stroke is impossible to be kept constant. This being the case, there encounter the following problems.
(1) Since it is difficult to reciprocate the reference mirror at high speed repeatedly, a repeated generation of interference fringes are difficult to obtain in a position where the optical path length of the measuring optical system is equal to the optical path length of the reference optical system. Consequently, during the alignment of the instrument and the correction of the refractive error with respect to the eye to be tested, since the alignment and the correction of the refractive error cannot be set to an optimum position while detecting the amplitude of light and darkness of the interference fringes by a photosensor, it is necessary to confirm the alignment and the correction of the refractive error first and then, the interference fringes are observed while moving the reference mirror.
However, in this method, even if the alignment and the correction of the refractive error are once set to the optimum position, it is no way to detect an actual generation of the interference fringes until the reference mirror is moved. Furthermore, there is a possibility that the alignment is displaced when the reference mirror moves to the position where the interference fringes are generated. Particularly, in the case where the eye to be tested is difficult to be measured because the eye is suffered from, for example, cataract, there are many cases that the interference fringes are not obtained. As a consequence, the probability of measurement is lowered.
(2) The frequency of a repetition of light and darkness caused by movement of the interference fringes observed by the photosensor is determined (2.times.(moving speed)/(wavelength)) from the wavelength of a light source and a moving speed (variation speed of the optical pathlength of the reference optical system) of the reference mirror. Therefore, if the moving speed of the reference mirror is fluctuated during a stroke thereof, the frequency of light and darkness caused by movement of the interference fringes to be observed is also varied. This makes it necessary to design the band width of the frequency characteristic of a light receiving circuit wide. However, if the band width of the frequency characteristic is designed wide, it becomes difficult to separate a noise component. In other words, it is difficult to realize a high signal to noise ratio (S/N). As a result, it is frequently unable to measure such an intraocular object to be tested as the eye suffered from cataract where losses in an amount of light reflected from the fundus of the eye to be tested are great and the signal to noise ratio (S/N) is bad.
The present invention has been accomplished in view of the above problems. It is therefore an object of the present invention to provide an intraocular length measuring instrument, in which the optical path length of a reference optical path can be converted at a high speed repeatedly.